There are numerous types of internal combustion engines in use today. Reciprocating piston internal combustion engines are very common in both two- and four-stroke configurations. Such engines can include one or more pistons reciprocating in individual cylinders arranged in a wide variety of different configurations, including “V”, in-line, or horizontally-opposed configurations. The pistons are typically coupled to a crankshaft, and draw fuel/air mixture into the cylinder during a downward stroke and compress the fuel/air mixture during an upward stroke. The fuel/air mixture is ignited near the top of the piston stroke by a spark plug or other means, and the resulting combustion and expansion drives the piston downwardly, thereby transferring chemical energy of the fuel into mechanical work by the crankshaft.
As is well known, conventional reciprocating piston internal combustion engines have a number of limitations—not the least of which is that much of the chemical energy of the fuel is wasted in the forms of heat and friction. As a result, only about 25% of the fuel's energy in a typical car or motorcycle engine is actually converted into shaft work for moving the vehicle, generating electric power for accessories, etc.
Opposed-piston internal combustion engines can overcome some of the limitations of conventional reciprocating engines. Such engines typically include pairs of opposing pistons that reciprocate toward and away from each other in a common cylinder to decrease and increase the volume of the combustion chamber formed therebetween. Each piston of a given pair is coupled to a separate crankshaft, with the crankshafts typically coupled together by gears or other systems to provide a common driveline and control engine timing. Each pair of pistons defines a common combustion volume or cylinder, and engines can be composed of many such cylinders, with a crankshaft connected to more that one piston, depending on engine configuration. Such engines are disclosed in, for example, U.S. patent application Ser. No. 12/624,276, which is incorporated herein in its entirety by reference.
In contrast to conventional reciprocating engines which typically use reciprocating poppet valves to transfer fresh fuel and/or air into the combustion chamber and exhaust combustion products from the combustion chamber, some engines, including some opposed-piston engines, utilize sleeve valves for this purpose. The sleeve valve typically forms all or a portion of the cylinder wall. In some embodiments, the sleeve valve reciprocates back and forth along its axis to open and close intake and exhaust ports at appropriate times to introduce air or fuel/air mixture into the combustion chamber and exhaust combustion products from the chamber. In other embodiments, the sleeve valve can rotate about its axis to open and close the intake and exhaust ports.
Internal combustion engines are typically required to perform over a wide range of operating conditions. In most instances, however, the optimum geometric compression ratio in the combustion chamber is not the same for each operating condition. To the contrary, the optimum compression ratio often depends on engine load, valve timing, and other factors. Variable valve timing provides some flexibility to optimize or at least improve engine performance based on load, fuel, temperature, humidity, altitude and other operating conditions. Combining variable valve timing with variable compression ratio (VCR), however, can further reduce pumping work losses by reducing intake throttling and optimizing the expansion stroke for improved power and efficiency at a given engine operating condition.
While some systems for varying valve timing have overcome the issue of complexity, systems for varying compression ratio in, for example, conventional internal combustion engines are generally very complex and, as a result, have not been widely adopted. In the case of opposed-piston engines, many of these are diesel engines which may not realize significant benefits from variable compression ratio.